The invention relates to an injection molding machine having a built-in load cell for detecting pressure applied to a screw.
Generally, electrically driven injection molding machines have a built-in load cell inside a screw drive mechanism to detect pressure to be applied to a screw. FIGS. 6 and 7 show the construction of a main portion of a conventional injection molding machine having a built-in load cell.
As shown in FIG. 6, an injection molding machine 60 has a guide coupling 53 which is slidably supported by a plurality of guide shafts 51 and to which linear movements from a ball screw mechanism 52 are transmitted. The machine 60 also includes a screw coupling 54 which is coupled to a rear end of a screw 55 and to which rotary movements are transmitted. The screw coupling 54 is rotatably supported by the guide coupling 53. Additionally, a washer-type load cell (e.g., pressure detector) 61, which includes an inner tubular portion 62, an outer tubular portion 63, and an intermediate distortion portion 64, is interposed between the screw coupling 54 and the guide coupling 53. The load cell 61 and the guide coupling 53 are secured to the screw coupling 54 by a bearing nut 56, together with a thrust bearing 58 and an angular ball bearing 59, both bearings 58, 59 interposing the load cell 61 and the guide coupling 53 therebetween.
With this arrangement, once pressure from the screw 55 is applied to the inner tubular portion 62 through the screw coupling 54 and the thrust bearing 58, the intermediate distortion portion 64 generates distortion which corresponds to the applied pressure because the outer tubular portion 63 is regulated by the guide coupling 53. This pressure is detected by distortion gages mounted on the intermediate distortion portion 64.
However, in the conventional injection molding machine 60, the torque of the bearing nut 56 acts on the outer tubular portion 63 through the intermediate distortion portion 64, thereby causing an initial distortion to be generated on the load cell 61, and the bearing height of the thrust bearing 58 and the thickness of the load cell 61 are decreased with an increasing injection force. As a result, a predetermined load or pre-load applied from the bearing nut 56 to the angular ball bearing 59 is decreased proportionally to such a decrease, thereby impairing linearity in the detection characteristic (output characteristic) of the load cell 61 corresponding to the magnitude of the injection force. Hence, pressure is incorrectly detected, and thus making corrections thereof is difficult.
Additionally, deviations from the zero point are caused not only by reversible variations in the pre-load applied from the bearing nut 56 due to changes caused by, for example, temperature variations and different thermal expansion coefficients among materials, but also by irreversible variations in the pre-load caused by the slackening of the bearing nut 56 due to repeated use. Such deviations reduce detection accuracy, reliability, and the like.
FIG. 7 shows an injection molding machine 70 (based on Japanese Kokai No. 27921/1989) in which the above conventional shortcomings are overcome. For clarity, the parts and components in FIG. 7 identical to those in FIG. 6 are designated by the same reference numerals. Injection molding machine 70 is constructed so that the load cell 61 is interposed between the guide coupling 53 and a ball nut 57 of the ball screw mechanism 52. With this arrangement, the influence of the pre-load applied from the bearing nut 56 is eliminated.
However, even though the injection molding machine 70 is free from the influence from the pre-load described above, the machine is affected by the frictional resistance of the guide coupling 53 and the like. That is, although the guide coupling 53 is supported slidably by the guide shafts 51, frictional and other error factors attributable to the guide shafts 51 exist, i.e., variable error factors such as thermal expansion coefficients and fixed error factors such as machining accuracy and parallelism during assembly of the guide shafts. Thus, the magnitude of friction also varies depending on the position of the guide coupling 53 and the elapsed time from the start of operation. As a result, the injection molding machine 70, like other conventional machines, suffers in its detection accuracy.
Further, the installation of the load cell 61 outside the guide coupling 53 requires a mounting screw hole, thereby making the load cell 61 unnecessarily large.